1. Field of the Invention
The present invention relates to inspection of articles, and in particular, to inspection of articles related to the manufacture of semiconductor devices. More specifically, the invention relates to the inspection of articles used in photolithography during the manufacture of semiconductor devices.
2. Description of the Related Art
Current demands for high density and performance associated with ultra large scale integration in semiconductor devices require submicron features, increased transistor and circuit speeds, and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring.
One important process requiring careful inspection is photolithography, wherein masks or “reticles” are used to transfer circuitry patterns to semiconductor wafers. Typically, the reticles are in the form of patterned chrome over a transparent substrate. A series of such reticles are employed to project the patterns onto the wafer in a preset sequence. Each photolithographic reticle includes an intricate set of geometric patterns corresponding to the circuit components to be integrated onto the wafer. The transfer of the reticle pattern onto the photoresist layer is performed conventionally by an optical exposure tool such as a scanner or a stepper, which directs light or other radiation through the reticle to expose the photoresist. The photoresist is thereafter developed to form a photoresist mask, and the underlying polysilicon insulators or metal layer is selectively etched in accordance with the mask to form features such as lines or gates.
From the above description, it should be appreciated that any defect on the reticle, such as extra or missing chrome, may transfer onto the fabricated wafer in a repeated manner. Thus, any defect on the reticle would drastically reduce the yield of the fabrication line. Therefore, it is of utmost importance to inspect the reticles and detect any defects thereupon. The inspection is generally performed by an optical system, using transmitted, reflected, or both types of illuminations. An example of such a system is the RT-8000™ series reticle inspection system available from Applied Materials of Santa Clara, Calif.
There are several different known algorithm methods for inspection of reticles. These methods include: “Die to Die” inspection, in which a die is compared to a purportedly identical die on the same reticle; or “Die to Database” inspection, in which data pertaining to a given die is compared to information in a database, which could be the one from which the reticle was generated. Another inspection method involves Die to golden dye which is a dye chosen as a reference for inspecting wafers. There also is a design rule based inspection, in which the dye has to fulfill some line width and spacing requirements, and feature shapes should fit predefined shapes. Examples of these inspection methods, and relevant apparatus and circuitry for implementing these methods, are described in various U.S. patents, including, inter alia, U.S. Pat. Nos. 4,805,123; 4,926,489; 5,619,429; and 5,864,394. The disclosures of these patents are incorporated herein by reference.
Known inspection techniques typically use imaging the articles with a large magnification onto a charge-coupled device (CCD) camera. The imaging technique requires the article to be illuminated. The brightness of the illuminating source is a key factor in the ability to speed the inspection by reducing the integration time of the camera. As the patterns on wafers get smaller, it becomes necessary to use smaller wavelengths in order to be able to detect the patterns. This is due to the fact that the physical resolution limit depends linearly on the illumination wavelength and due to interference effects which require that the inspection be done at a wavelength similar to the one used in the lithographic process. As the wavelengths get smaller, conventional incoherent light sources like filament lamps or gas discharge lamps do not have enough brightness, and the light sources of choice become short wavelength lasers. The coherence of the laser, together with the roughness and aberrations of the surfaces as well as the patterned article along the light path, creates an artifact known as speckle, which is a noisy pattern over the image of the article.
Speckle causes problems in detection of the surfaces of articles being inspected and causes false alarms because of the non uniformity of the light pattern hitting the detector. Detection accuracy is degraded. Also, images taken of inspected articles are degraded. The problem is an acute one in this type of article inspection, because the power provided by coherent light is essential, among other reasons, as a result of losses stemming from the detection process.
The just-discussed problems are not unique to inspection of masks, photomasks, and reticles. There are known wafer inspection techniques which employ coherent illumination. In such systems, speckle can have an adverse impact on yield and performance of the resulting devices, and so also must be addressed with great care. Examples of known wafer inspection systems employing coherent illumination are shown in U.S. Pat. Nos. 5,699,447 and 5,825,482. The disclosures of these patents also are incorporated herein by reference.
When such systems are used to inspect patterned wafers, the speckle phenomenon can arise, if the spot size used for illumination is not much smaller than an element of a pattern on the wafer. However, in some circumstances, such as oblique illumination (in which the coherent light source is directed to the wafer at an angle), the spot size will be sufficiently large to cause speckle. Reducing the spot size will reduce system throughput and will require working at a wavelength that is smaller and different from the one used for imaging the article for example during the lithographic process. Consequently, as can be appreciated, there is a tradeoff between enduring speckle and optimizing detection sensitivity/throughput. Therefore, it would be desirable to solve the speckle problem, and thus enable the use of an increased spot size, and thus improve throughput.
A comprehensive description of speckle phenomena can be found in T. S. McKechnie, Speckle Reduction, in Topics in Applied Physics, Laser Speckle and Related Phenomena, 123 (J. C. Dainty ed., 2d ed., 1984) (hereinafter McKechnie). As discussed in the McKechnie article, speckle reduction may be achieved through reduction in the temporal coherence or the spatial coherence of the laser light. There have been various attempts over the years to reduce or eliminate speckle.
Another article, citing the above-mentioned McKechnie article and addressing the same issues, B. Dingel et al., Speckle reduction with virtual incoherent laser illumination using a modified fiber array, Optik 94, at 132 (1993) (hereinafter Dingel), mentions several known methods for reducing speckle based on a time integration basis, as well as based on statistical ensemble integration. With respect to the time integration methods, involving scanning of various planes of the imaging system and generating uncorrelated image patterns to be integrated by an image detector, the article identifies some possible drawbacks, such as a long integration time, or introduction of additional optical systems to support the scanning process.
Among the methods involving a reduction in the coherence of the beam, both Dingel and McKechnie discuss the introduction of a dispersing element, such as a grating, a screen mesh, or a moving diffuser, by itself or in combination with another rotating diffuser, into the path of the illuminating beam so as to produce a random phase modulation over the extent of the light beam. Other known techniques involve the passage of a pulsed laser beam through a carbon disulfide cell and further through an unaligned fiber optic bundle, or the use of liquid crystals interposed in the path of the light beam, the crystals being moved by electric field excitation.
However, as the reticles become smaller in size and the patterns shrink, it becomes more difficult to manufacture them with no fine anomalies and small defects. With diffraction effects, the detection becomes more complicated as well. Therefore, the danger exists that small defects may go undetected, which could cause problems in the related wafer manufacturing process. One proposed solution to the current situation involves the use of a laser light source emitting low wavelength laser beams, preferably in the deep ultraviolet (UV) region to illuminate the article. The laser source would preferably emit short pulses of light, with a preferred range of 5-50 nanoseconds. None of the methods and systems discussed above is equipped to offer a high level of speckle reduction for low wavelength laser beams so as to ensure accurate defect detection. Also, the above methods and systems do not provide a reliable solution for short laser pulses because of inadequate moving speed of the dispersing elements.
As alluded to previously, speckle also is a known phenomenon in the wafer inspection area. U.S. Pat. No. 5,264,912, the disclosure of which is incorporated herein by reference, identifies this problem, and provides some proposed solutions. However, as with other known and proposed speckle reduction techniques, these proposed solutions do not address particular problems resulting from the need to work with extremely small features, and the consequent need to employ coherent illumination sources with very low wavelengths.
Speckle reduction devices, interposed in the light path between the article surface and the detector, also can be expensive. For example, interposing a fiber bundle in accordance with one of the techniques described above could require as many as 10,000 fibers with different properties, such as length, in the bundle. These fiber bundles would be extremely large in size, and consequently would be expensive. It would be desirable to find a solution that did not need so many fibers.
A similar problem would pertain with respect to the use of a diffraction grating. The finer and larger in size the grating, the more expensive it would be. It would be desirable to find a solution, using diffraction gratings, but which did not require exceedingly fine gratings.
As can be appreciated from the foregoing discussion, there is a need in the art for a method and system for reducing speckle when inspecting articles using pulsed laser beams at low wavelengths, including the deep UV region.